Review of Medical Physiology - William F. Ganong 2002

Functions of the Nervous System
Hearing and Equilibrium
Anatomical Characteristics

The External and Middle ear

The auricle (pinna) of the External ear directs sound waves into the external auditory meatus. In some animal species, the ears can move to locate the source of sounds, acting much like radar dishes. From the external auditory meatus, the external auditory canal leads to the tympanic membrane (Fig. 9-1). The middle ear is an air-filled cavity within the Temporal bone that communicates with the nasopharynx via the auditory (Eustachian) tube, and through it with the external environment. The auditory tube is normally closed, but opens during swallowing, chewing, and yawning, which equalizes the air pressure on both sides of the tympanic membrane. The middle ear contains three auditory ossicles: the malleus, incus, and stapes. The handle of the malleus attaches to the inner surface of the tympanic membrane. The HEAD of the malleus abuts the wall of the Inner ear, and its short process connects to the incus, which in turn articulates with the head of the stapes. The stapes gets its name from its resemblance to a horse stirrup. The base (footplate) of the stapes is connected by the annular ligament to the margin of the oval window (Fig. 9-2). The middle ear also houses two small skeletal Muscles: the stapedius and the tensor tympani. Contraction of the tensor tympani pulls the handle of the malleus medially, dampening the vibrations of the tympanic membrane, whereas contraction of the stapedius pulls the footplate of the stapes out of the oval window.

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Fig. 9-1. The human ear. For better visualization, the auricle is slightly reflected and the middle ear muscles have been removed.

Fig. 9-2. Schematic representation of the auditory ossicles and the pathway converting Movements of the tympanic membrane into fluid waves in the inner ear. Wave dispersion occurs through the round window. The movements of the ossicles and the membranous labyrinth, as well as the round window, are indicated by dashed lines.

The Internal Ear

The internal ear (labyrinth) consists of two parts, one nested inside the other. The osseous (bony) labyrinth comprises a system of channels and cavities within the petrous part of the temporal bone. Suspended within these channels, bathed in a fluid called perilymph, is the membranous labyrinth (Fig. 9-3), which mirrors the shape of the bony labyrinth and is filled with endolymph. The spaces filled with endolymph and perilymph do not communicate with each other.

The Cochlea

The cochlear portion of the labyrinth is a tubular Structure (about 35 mm long in humans) that makes 2³/₄ turns. Throughout its length, the basilar and vestibular (Reissner's) membranes divide the cochlea into three parallel compartments (scalae) (Fig. 9-4). The upper scala vestibuli and lower scala tympani contain perilymph and communicate with each other at the apex of the cochlea through a small opening called the helicotrema. At the Base of the cochlea, the scala vestibuli terminates at the oval window, which is sealed by the footplate of the stapes. The scala tympani ends at the round window—an aperture in the medial wall of the middle ear covered by the elastic secondary tympanic membrane. The scala media (cochlear duct), the middle chamber, is part of the membranous labyrinth; it has no direct connection with the scala vestibuli or scala tympani and is filled with endolymph (see Figs. 9-3, 9-4).

Fig. 9-3. Human membranous labyrinth with enlarged structures containing Hair Cells (reproduced with permission from Hudspeth AJ: How the ear's works work. Nature 1989:34:397).

The Organ of Corti

Located on the basilar membrane, the organ of Corti contains the hair cells, which serve as the auditory receptors. This spiral-shaped organ extends from the apex of the cochlea to its base. The apical processes of the hair cells pierce a dense fibrous plate supported by pillar cells, or rods of Corti (see Fig. 9-4). The hair cells are arranged in four rows: three rows of outer hair cells are situated lateral to the inner tunnel formed by the pillar cells, and one row of inner hair cells lies medial to it. Each human cochlea contains approximately 20,000 outer hair cells and 3,500 inner hair cells. The rows of hair cells are covered by a thin, viscous, elastic tectorial membrane, into which the tips of the stereocilia of the outer hair cells are embedded. The stereocilia of the inner hair cells do not reach the tectorial membrane. The Cell bodies of afferent Neurons, whose processes wrap around the bases of the hair cells, form the spiral ganglion located within the modiolus—the bony core around which the cochlea is wound. About 90–95% of spiral ganglion neurons innervate the inner hair cells, whereas only 5–10% provide innervation to the much more numerous outer hair cells, with a single neuron typically innervating several such cells. Conversely, most efferent fibers of the auditory nerve (see below) terminate on the outer hair cells. The axons of afferent neurons innervating the hair cells form the cochlear division of the Vestibulocochlear nerve and terminate in the dorsal and ventral cochlear nuclei of the Medulla Oblongata. The total number of afferent and efferent fibers in each auditory nerve is approximately 28,000.

Within the cochlea, the hair cells form tight junctions with adjacent phalangeal cells, creating a barrier that prevents endolymph from reaching the bases of the hair cells. At the same time, perilymph leaks from the scala tympani through the basilar membrane, filling the inner tunnel and the spaces around the bases of the inner and outer hair cells. Similar tight junctions establish the characteristic arrangement of hair cells in other Regions of the inner ear. The hair cell processes are immersed in endolymph, whereas their basal regions are bathed in perilymph.

Central Auditory Pathways

From the cochlear nuclei, auditory impulses travel via various pathways to the inferior colliculi, which serve as integration centers for auditory Reflexes, and through the medial geniculate bodies of the thalamus to the auditory cortex. Other impulses project to the reticular formation (Fig. 9-5). Information from both ears converges at the superior olivary complexes, and the majority of neurons at higher levels receive bilateral input from both sides of the head. The primary auditory cortex, or Brodmann area 41, is located in the superior part of the temporal lobe. In humans, it lies within the Sylvian fissure (see Fig. 7-4) and is normally hidden from The surface of the Brain. Within the primary auditory cortex, most neurons are binaural, but There are also alternating bands of cells excited by contralateral input and inhibited by ipsilateral input. There are several secondary auditory areas, analogous to the multiple somatic sensory areas (see Chapter 7). Auditory association areas lie adjacent to the primary areas and cover a considerable expanse (see below). The olivocochlear bundle is a prominent tract of efferent fibers in each auditory nerve that originates from both the ipsilateral and contralateral superior olivary complexes and terminates predominantly around the basal regions of the outer hair cells in the organ of Corti.

Fig. 9-4. Top: Cytology/practical/72.html">Cross section of the cochlea showing the Topography of the organ of Corti and the scala vestibuli, scala media, and scala tympani. Bottom: STRUCTURE OF THE organ of Corti from the lower turn of the cochlea; DC = Deiters' cells (outer phalangeal cells) that provide structural support for the outer hair cells (reproduced with permission from Pickles JO: An Introduction to the Physiology of Hearing, 2nd ed. Academic Press, 1988).

Fig. 9-5. Simplified diagram of the main Brainstem auditory pathways, posterior view. The Cerebellum and Cerebral Cortex have been removed.

Semicircular Canals

On both sides of the head, the semicircular canals are oriented perpendicularly to one another in three spatial planes. Membranous canals, surrounded by perilymph, are suspended within the bony canals. The receptor structure—the crista ampullaris—is located within the widened region (ampulla) of each membranous canal. Each crista consists of hair cells and supporting cells, capped by a gelatinous structure known as the cupula, which spans the ampulla (Fig. 9-6). The hair cell stereocilia are embedded in the cupula, while their bases form synapses with the efferent fibers of the vestibular division of the vestibulocochlear nerve.

Utricle and Saccule

Within each membranous labyrinth, an otolith organ called the macula is located on the floor of the utricle. Another macula is situated in a semi-vertical orientation on the wall of the saccule. Each macula consists of hair cells and supporting cells, overlain by an otolithic membrane containing calcium carbonate crystals called otoliths (see Fig. 9-3). The otoliths, also known as otoconia or ear dust, range in humans from 3 to 19 μm in length and have a density greater than that of the endolymph. The hair cell stereocilia are embedded in the otolithic membrane. Nerve fibers innervating the bases of the macular hair cells join the fibers from the cristae ampullares to form the vestibular division of the vestibulocochlear nerve.

Neural Pathways

The cell bodies of the 19,000 neurons innervating the crutae and maculae of each ear are located in the vestibular ganglion. Each vestibular nerve terminates in the ipsilateral four-component vestibular Nucleus and in the nodulofloccular lobe of the cerebellum (Fig. 9-7). Fibers from the semicircular canals terminate predominantly in the superior and middle PARTS OF THE vestibular nucleus and project mainly to the nuclei controlling Eye Movements. Fibers from the utricle and saccule terminate mostly in the lateral part of the vestibular nucleus (Deiters' nucleus), which projects to the Spinal Cord. They also terminate in the descending nucleus, which projects to the cerebellum and reticular formation. The vestibular nucleus also projects to the Thalamic region, and from there to two areas of the primary somatosensory cortex.